(1) Field of the Invention
The present invention relates generally to range measurement systems (RMS) for determining the relative distance between two or more undersea and surface vessels and deals more particularly with a digital range measurement system (DRMS) to acoustically measure and record the ranges between undersea and surface vessels by measuring sound propagation delay through the ocean by using digital signal processing (DSP) techniques to determine the frequency of a detected signal at a predetermined time to identify the source and the range to the source at the time of detection.
(2) Description of the Prior Art There are numerous naval and oceanic related exercises or projects involving Sonar at sea testing with two or more submarines or surface ships which are concerned with the relative distances between test platforms during the exercise. There is no measurement device independent of the test platform's Sonar system which can provide the desired range information. Therefore when testing the performance of the ship's Sonar or tracking capability, test personnel have no basis other than the test plan to compare range predictions with and to determine the overall test results accuracy.
Prior to the present invention, such distances were determined using a range measurement system (RMS). The known RMS experiences a number of shortcomings among which are limitations in its detection capabilities and lack of flexibility to accommodate the varying test requirements. Another serious drawback of the known RMS is that the detection circuit is implemented completely with hardware and comprises three phase-locked loop circuits which are permanently tuned to accept three specific, given frequencies. Inherent in this design is the lack of flexibility to change any one or all of the detect frequencies. Furthermore, the known RMS suffers in its capability to extract and detect vary low level signals which could be Doppler shifted in frequency such that the RMS fails to detect and operate directly in many common long range exercises.
As known, conventional Sonar systems typically depend upon a return pulse which is transmitted to and reflected from another vessel or object so that return signal is received at the point of origination.
U.S. Pat. No. 4,513,401 issued to Ottsen et al on Apr. 23, 1985 describes a marine cable locating system wherein a plurality of acoustic transponders are placed at spaced intervals along cable with each transponder transmitting an acoustic signal at a unique characteristic frequency so that the precise location of each transponder can be accurately determined from slant range measurements as the vessel follows a parallel path and from water depth measurements as the vessel crisscrosses the cable. Each transponder is interrogated from a unit on the vessel and transmits an acoustic signal in response thereto at each time the transponder is interrogated from which a set of loci is plotted for all possible positions in order to identify the exact position of the transponder.
Pat. No. 4,516,226 issued to Peynaud et al on May 7, 1985 discloses an acoustic positioning system wherein an interrogator and a number of transponders are moored to the seabed in known positions to define a fixed reference system. The interrogator transmits a first frequency signal which in turn causes each of the transponders to transmit a unique associated frequency signal in response thereto. The acoustic frequency signal from the interrogator and from the transponders are received by an acoustic array located at the vessel for which it is desired to positionally determine the coordinates X and Y of the vessel within the fixed reference system. The location is determined by utilizing the transit time of the signal from the interrogator and the transit time of the signal from the transponder which is transmitted in response to the interrogation signal. The time that the interrogator transmits its signal is also transmitted to the surface vessel for use in determining its position in the case of an undersea vessel, it is contemplated that the time of the periodic transmissions by the interrogator are synchronized with an onboard clock so that the time of transmission is known. This patent discloses that transmission of acoustic pulse can be made from a source known geographically (triangulation is used to determine the X, Y coordinate of the vessel within the fixed reference system).
Neither the prior art nor the known range measuring systems disclose the concept of transmitting a characteristic, unique frequency signal at a predetermined time synchronized relative to a real-time clock whereby the time detection delay is used to determine the range or distance between two or more vessels without the need of transponders which are in a known position defining a fixed reference system.
The DRMS employing the present invention overcomes the known problems with RMS by implementing the detection and transmission process with software rather than hardware as previously done. The implementation of the digital range measuring system utilizing software algorithms inherently provides greater flexibility over hardware implementation since changing the detection and transmission frequency requires only certain program variables to be changed. The DRMS also provides the ability of a user to set the detection and transmission frequency in real time. The DRMS of the present invention utilizes a state of the art digital signal processing processor with dedicated programming which runs concurrently with the software of a personal computer to perform the detection of a signal with a known predetermined frequency and transmission functions as explained in further detail hereinbelow. The process of detection with the DRMS of the present invention provides a more accurate and reliable detecting of incoming signals with varying Doppler shifts and very low level signals.